OBJECTIVE: To improve prediction of ovulation in normal cycles. DESIGN: Collection of women's characteristics and their menstrual cycles. Monitoring and analysis of time relationships between several indicators of ovulation: transvaginal ultrasonography, cervical mucus, basal body temperature, urinary luteinizing hormone, and ratio of urinary estrogen to progesterone metabolites. SETTING: Each of eight natural family planning clinics was to study 12 women for at least three cycles. POPULATION: 107 normally fertile and cycling women aged 18-45. METHODS: Daily measurements of urinary LH, FSH, estrone-3-glucuronide, and pregnanediol-3alpha-glucuronide. Basal body temperature recording and cervical mucus checking. Transvaginal ultrasound examination of the ovaries. MAIN OUTCOME MEASURES: Delays between the expected day of ovulation according to the luteinizing hormone peak or to ultrasound evidence and the expected days according to the other indices of ovulation. RESULTS: Ultrasonography was able to show evidence of ovulation in 283 out of 326 cycles. The average time lag between luteinizing hormone peak and ultrasound evidence was less than one day (+0.46), but premature and late luteinizing hormone-expected date of ovulation were observed in nearly 10% and 23% of cycles, respectively. Basal body temperature rise was observed in 98% of cycles. Cervical mucus peak symptom, rapid drop in the ratio of urinary metabolites, and luteinizing hormone initial rise were all close to ultrasonographic evidence in more than 72% of cycles. CONCLUSIONS: For accuracy and practical reasons, the cervical mucus peak symptom, the ratio of urinary metabolites, and luteinizing hormone initial rise might be better indices of ovulation than the luteinizing hormone peak.